The present invention relates to the coagulation of latex, as the term is broadly defined. Broadly, a latex is an emulsion of polymeric materials. Such emulsions are often formed in a polymerization process known as emulsion polymerization. Once formed in the emulsion, the smaller particles must be coagulated into larger particles and separated from the surfactant in water solution which carries the emulsion.
The two most commonly used commercial coagulation processes are shear and chemical coagulation. Shear coagulation involves mechanical agitation of the emulsion. While coagulation is effective, the water remaining after coagulation is completed is opaque and cloudy. It presents a serious disposal problem. It cannot readily be reused due to the emulsified polymer remaining.
Chemical coagulation is also effective. The residual water is clear. However, it is contaminated with the coagulating chemicals and also presents a disposal problem. The contaminating chemicals prevent one from reusing the surfactant solution in subsequent emulsification.
Freeze coagulation in an embodiment which simulates commercial ice making equipment has been used on a small scale. It involves immersing a chilled freezing surface into the emulsion to freeze the emulsion, thereby causing the polymer to coagulate. After thawing, the polymer is coagulated and the residual surfactant water solution is crystal clear and free of chemical contaminants. Hence it can be readily reused in further emulsification polymerization.
However, the polymer, especially if it constitutes or includes an elastomeric component and is high enough in solids to be of commercial interest (higher than 20% solids, but preferably above 30% solids and most preferably above about 35% solids), has a tendency to adhere to the freezing surface. It has to be scraped off of the freezing surface. German Pat. No. 32 30 128 discloses a scraping mechanism for scraping the coagulated polymer from the freezing surface used in a freeze coagulation process.
Others have tried to obviate this difficulty by eliminating the freezing surface. In such an approach, the emulsion is introduced directly into a refrigerant such as freon. The frozen, coagulated mass is then thawed and the clear water separated from the coagulated product. The drawbacks to this method include the expense of the refrigerant material as well as dealing with the vapors thereof, and difficulty in removing traces of the refrigerant from the product.
These drawbacks have relegated freeze coagulation to a "back seat" vis-a-vis other coagulation techniques.